Resistance against erosion wear is normally related to the hardness of the wear part. Some articles are subject to solid particle erosion in which particles of various sizes and hardness are propelled at various angles against the surface of the articles. For example, a car traveling in the desert during a wind storm will encounter various size solid particles of sand traveling at various velocities hitting the car. If the size of the particles is large and the velocity of the particles is high, the coating on the car could be chipped or pitted.
In turbomachines which operate in a dust environment, this solid particle erosion is a severe problem. Physical and chemical vapor deposited coatings, such as titanium nitride coatings and zirconium nitride coatings, have been used to provide a protective layer having good hardness characteristics. These coatings have been found to have good erosion resistance to Al2O3 and SiO2 particles at both high and low impact angles. Although these coatings have high hardness characteristics, they exhibit inherently brittle behavior and their erosion resistance at normal impact decreases markedly with increasing hardness and particle size of the erodent.
It has been observed that dynamic impact of solid particle erodents onto a coated surface of an article can form lateral and/or median cracks around the impact site. Median cracks are responsible for the strength degradation of the material while lateral cracks, which grow from the center of impact parallel to the substrate surface and then propagate through the coating surface, account for most of the material loss during solid particle impact erosion. The solid particle impact erosion of these coatings at a 90° impact angle is due primarily to brittle fracture.
Thin coatings are more suspectible to spalling and exposure of the substrate which may lead to premature failure of the article. When coatings applied by conventional techniques are exposed to particle impact, pinholes and/or lateral spalling pits generally result in the coating. Once the coating material is cracked, additional impact by even relatively small particles will cause furrowing or grooves in the coating material. In a turbomachine, this furrowing can greatly effect the overall performance of the turbomachine.
Toughness and hardness are the dominant properties controlling the erosion behavior. Higher hardness is believed to increase erosion resistance at both low and high impingement angles while higher toughness reduces the vulnerability to brittle fracture and markedly increases 90° erosion resistance. An erosion resistant coating needs to be simultaneously hard and tough. However, hardness and toughness are generally in opposition in hard materials. Higher hardness is usually associated with greater brittleness. Multilayer hard compound materials have been found to have simultaneously high hardness and high toughness. The high hardness is an inherent property of hard compounds and the high toughness is attributed to the formation of a coherent or partly coherent interface boundary between two different hard compound layers.
It has been found that conventional erosion resistant coatings can be used to protect compressor blades and vanes against fine sand erosion such as coarse AZ road dust with a median particle size of approximately 40 microns, a maximum particle size of approximately 170 microns, and a minimum particle size of approximately 0.85 microns. See, for example, U.S. Pat. No. 5,071,693 which describes a multilayer coated substrate having good erosion and/or wear resistance characteristics to fine solid particle impact.
However, conventional erosion resistant coatings are not very well performing in the case of large particle erosion media such as 40 mesh Sakrete sand with a median particle size of 230 microns. When eroded with large particle media, conventional coatings are shattered and large pits are created. Subsequently, fine particle media can attack the edges of the large pits and cause acceleration of the erosion.
There continues to be a need in the art for erosion resistant coatings that perform well in the case of both small and large particle erosion media. The erosion resistant coatings should perform well, for example, in the case of large particle erosion media such as 40 mesh Sakrete sand with a median particle size of 230 microns.